package com.codeforces;

import java.util.Scanner;

public class _317A {

	static long x, y, m, tmp, depth;
	static Scanner scanner = new Scanner(System.in);

	public static void main(String[] args) {
		depth = 0;
		x = scanner.nextLong();
		y = scanner.nextLong();
		m = scanner.nextLong();
		if (x < y) {
			tmp = x;
			x = y;
			y = tmp;
		}
		if (m > x) {
			if (x <= 0) {
				System.out.println("-1\n");
			}
			else {
				if (y < 0) {
					tmp = -y;
					tmp = tmp / x;
					y = y + x * (tmp+1);
					depth = (tmp+1);
				}
				int d = 0;
				do {
					tmp = y;
					y = x;
					x = x + tmp;
					d++;
				} while(x < m);
				System.out.println((depth + (long) d) + "\n");
				//cout << depth + (long long) d<< "\n";
				//1100087778366101931
			}
		}
		else {
//			cout << "0\n";
			System.out.println("0\n");
		}
	}

}
